A conventional power semiconductor device includes an active portion and an edge termination portion that surrounds the active portion. The edge termination portion can contribute to removal of undesired leakage current, undesired electric field crowding, or accumulated electrostatic charges in the active portion, and withstanding of high reverse voltages.
The active portion mainly includes a plurality of transistors or rectifiers that are electrically connected in parallel. More specifically, the transistors are formed with a plurality of gates, wells, and alternately arranged n-type and p-type semiconductor pillars. The edge termination portion typically includes alternate n-type and p-type semiconductor pillars like the active portion. A pitch between two adjacent ones of the p-type and n-type semiconductor pillars of the edge termination portion has to be precisely controlled. When the pitch is too large, the breakdown voltage of the edge termination portion is smaller than that of the active portion. Then the edge termination portion would fail to withstand the breakdown voltage of the power semiconductor device, and would be likely to break down prior to the active portion breaking down. When the pitch is too small, the depletion region between two adjacent ones of the p-type and n-type semiconductor pillars of the edge termination portion cannot be enlarged to withstand a relatively high breakdown voltage.
Another edge termination portion is fabricated in a diode structure that is formed very closely to the top surface by extension of a junction between a p-type semiconductor layer and an n-type semiconductor layer of the active region. When doping concentrations of the n-type and p-type semiconductor layers are reduced to increase a depletion region between the n-type and p-type semiconductor layers, the electric field strength is reduced. Hence, the edge termination portion is able to withstand a high reverse voltage. When the doping concentrations of the n-type and p-type semiconductor layers of the conventional edge termination portion are increased, the electric field intensity is increased thereby, whereas the depletion region is decreased and the electric force lines are unevenly distributed at the edge termination portion. Thus, the edge termination portion can only withstand a relatively low breakdown voltage. Furthermore, a depth of the junction measured from the top is relatively small and a relatively large area is needed for the optimal design of the edge termination portion, which requires that the chip size be relatively large.